Light emitting device with transparent conductive group-iii nitride layer

ABSTRACT

A group III-nitride semiconductor device comprises a light emitting semiconductor structure comprising a p-type layer and an n-type layer operable as a light emitting diode or laser. On top of the p-type layer there is arranged an n+ or n++-type layer of a group III-nitride, which is transparent to the light emitted from the underlying semiconductor structure and of sufficiently high electrical conductivity to provide lateral spreading of injection current for the light-emitting semiconductor structure.

The invention relates to the field of semiconductor-based light emittersand in particular to light emitters based on nitrides of group IIIelements. It relates to the devices themselves and also to methods formanufacturing such devices. Such devices can be or include, e.g., LEDsor lasers.

Definition

“Vertical” in the present description denotes directions in which layergrowth takes place. In usual wafer level manufacturing techniques,vertical directions are aligned perpendicular to the plane described bythe wafer.

“Lateral” in the present description denotes directions which areperpendicular to vertical directions, i.e. denotes directions parallelto the wafer plane.

At the end of the last century, high efficiency light emitting diodesand laser diodes with UV and visible emission have been developed whichare based on layers of p-type Mg doped nitrides of group-III elements.For brevity, we shall use “III-nitride” as a shorthand for “nitride ofone or more group-III elements.” Such devices have nowadays becomeubiquitous and find use, e.g., in solid-state lighting, in data storage,in projection applications in industrial applications. Therefore,devices herein described can find applications in these fields, too.

However, Mg-doped III-nitride layers are complicated to deal with due tothe fact that the dopant, i.e. Mg, has a high activation energy (lowhole concentrations despite high Mg concentrations) and sinceactivation/passivation of the Mg dopant is accomplished by heattreatments and by interaction with hydrogen, respectively. Mg-dopedIII-nitride layers frequently have a high resistivity, e.g., inlight-emitting p-i-n junctions. And control of the quality of metallicor semitransparent conductive contacts to the layers is difficult.

An object of the present disclosure is to describe ways of achieving aparticularly high conversion efficiency, i.e. having a particularly highlight output per inputted electrical energy, as well as to describecorresponding devices.

Another object of the present disclosure is to describe ways ofachieving a particularly high-intensity light emission, as well as todescribe corresponding devices.

Another object of the present disclosure is to describe ways ofachieving very homogeneous current spreading, in particular lateralcurrent spreading, of injection currents for light emission, as well asto describe corresponding devices.

Another object of the present disclosure is to describe ways ofachieving particularly low contact resistances towards electricalcontacts of devices and/or towards a semiconductor layer of alight-emitting semiconductor structure of a device, as well as todescribe corresponding devices.

Another object of the present disclosure is to describe ways ofachieving a high transparency of layers though which light is emittedfrom a light-emitting semiconductor structure, as well as to describecorresponding devices.

Another object of the present disclosure is to describe ways ofachieving a high light emission density, as well as to describecorresponding devices.

In addition, methods for manufacturing respective devices shall bedescribed.

Further objects and various advantages emerge from the description andembodiments below.

At least one of these objects is at least partially achieved by devicesand methods according to the patent claims.

A device is described which comprises

-   -   a semiconductor structure which is operable to emit light and        which comprises a first layer and a second layer; and in        addition,    -   a transparent layer of a nitride of one or more group-III        elements, which is transparent to light emitted from the        semiconductor structure.

The transparent layer is present on the first layer. Thus, thetransparent layer and the first layer can share a common interface.E.g., the transparent layer and the first layer can be in directcontact, e.g., the transparent layer can be deposited directly on thefirst layer. The transparent layer can be present on the first layer(such as on a portion of the first layer) with no further layer presentbetween them (in the range of said portion).

Furthermore, the transparent layer can have a low resistivity.

The transparent layer can have a resistivity so low that it can beconsidered an electrically conductive layer. The resistivity of thetransparent layer can be, e.g., below 5·10⁻⁴ Ωcm or even below 2·10⁻⁴Ωcm. In instances, the resistivity of the transparent layer is at most1·10⁻⁴ Ωcm.

The electrical conductivity of the transparent layer can exceed theconductivity of the first layer by a factor of at least 10, e.g., of atleast 50. In instances, the electrical conductivity of the transparentlayer exceeds the electrical conductivity of the first layer by a factorof at least 100.

The very high conductivity (or equivalently: low resistivity) of thetransparent layer can result in excellent (lateral) current spreadingproperties of the transparent layer. Holes in p-doped III-nitride layershave large effective mass, which results in a typically lower mobilityif compared to n-doped layers. Furthermore, Mg (which apparently is theonly known p-dopant to date) is a deep acceptor, and only a smallfraction of the atoms in the crystal are ionized to provide holes in thevalence band. And in addition, there is a physical limit to the quantityof Mg that can be introduced into a III-nitride layer above which themagnesium does not function as p-dopant anymore, because above a certainlimit, the impurity atoms form aggregates and other large defects sothat at least a portion of the introduced Mg does not function asp-dopant anymore. Thus, according to today's understanding, it is notpossible to make very low resistivity p-layers in III-nitride materials.Low resistivity n-doped III-nitride layers, however, are wellproducible.

The transparent layer can be provided for injecting charge carriers intothe first layer or for receiving charge carriers from the first layer.

The transparent layer can be a layer of a nitride of, e.g., one or moreof Ga, Al, In.

The transparent layer can be transparent to UV light and/or to visiblelight.

The light emittable from the semiconductor layer can be UV light orvisible light.

The device can be a light emitting device. It can be, e.g., an edgeemitting semiconductor device such as an edge emitting laser, a verticalcavity surface emitting laser (VCSEL), a light emitting diode (LED), anRCLED (resonant cavity LED).

The semiconductor structure can be, e.g., any known light-emittingsemiconductor structure, such as a p-i-n layer stack (p-doped layer,intrinsic layer, n-doped layer) or a p-n layer stack (p-doped layer,n-doped layer).

In some embodiments, the first and second layers are dopedsemiconducting layers.

The first layer can be of a first doping type and the second layer of asecond, opposite doping type. I.e. the first layer can be p-type (i.e.,p-doped), while the second layer is n-type (i.e., n-doped). Or, viceversa, the first layer can be in other embodiments n-type (i.e.n-doped), while the second layer is p-type (i.e., p-doped).

Throughout the present description, mostly the case is described thatthe first layer is p-type and the second layer is n-type. And, inaddition, mostly, Mg is mentioned as p-type dopant for the first layer.However, it can be vice-versa, too, and corresponding n-type dopants canbe, e.g., Si or O or Ge.

The semiconductor structure has an active region in which the light isgenerated. In case of a p-i-n structure, the active region coincideswith the i-layer; in case of a p-n structure, the active regioncoincides with the interface between the p-doped and the n-doped layer.

In some embodiments, the first layer is a III-nitride. E.g., it cancomprise the same III-nitride as the transparent layer does. E.g., ifthe transparent layer is made of Al_(x)Ga_(y)In_(1-x-y)N (with 0≦x≦1;0≦y≦1), also the first layer can be made of Al_(x)Ga_(y)In_(1-x-y)N(with identical x and y). Also, the second layer and/or (if present), toan i-layer between the first and second layer can be a III-nitridelayer.

In some embodiments, the semiconductor structure is an epitaxialstructure, e.g., a homoepitaxial layer stack. E.g., the first layer, thesecond layer and, if present, an i-layer between the first and secondlayer can all be layers made of Al_(x)Ga_(y)In_(1-x-y)N (with 0≦x≦1;0≦y≦1), with different or with the same x and y, but usually differingin their respective dopants and/or dopant concentrations.

Furthermore, e.g., in addition, the transparent layer can be anepitaxial with the first layer, in instances even a homoepitaxial layer.It can be of the same doping type or of the opposite doping type.

Epitaxial layers can show a high crystal quality and can thus have lowresistivities and high degrees of transparency. Moreover, layer-to-layerinterface resistances can be particularly low.

However, in general, the transparent layer can alternatively have adifferent crystallinity and, e.g., be a polycrystalline layer.

In some embodiments, light produced in the semiconductor structure isemitted through the transparent layer. This can facilitate themanufacture of top-emitting devices such as VCSELs.

In some embodiments, the device comprises a substrate, the second layerbeing arranged between the substrate and the first layer. It is,however, also possible to manufacture the device without an additionalsubstrate and/or to provide a substrate (with the second layer beingarranged between the substrate and the first layer) during somemanufacturing steps and to remove the substrate in a later processingstep.

The semiconductor structure can be produced in any known way. E.g., thesemiconductor structure can be produced by MOVPE (Metal Organic VaporPhase Epitaxy). Or, at least the first layer is produced by MOVPE. Ifthe semiconductor structure comprises an i-layer, at least the i-layerand, e.g., also the first layer, can be produced by MOVPE.

MOVPE is also known as, MOCVD (Metal Organic Chemical Vapor Deposition).

When a p-doped (such as an Mg doped) p-layer is produced by MOVPE, e.g.,as the first layer, the dopant can be present in a passivated state,e.g., due to the presence of H (hydrogen in the layer). In order toactivate the dopant (e.g., Mg), a heat treatment (also referred to as“annealing”) can be applied, e.g., by heating to a temperature between400° C. and 900° C.

In some embodiments, the transparent layer is deposited after anactivation, step has been accomplished in which passivated dopantpresent in the first layer is activated, e.g., in an above-describedway.

For depositing the transparent layer, a deposition technique can be usedwhich is different from the deposition technique used for depositing thefirst layer.

This can avoid contributing to a passivation of dopant present in thefirst layer, e.g., if the dopant can be passivated by H, a technique canbe used in which the first layer is not exposed to H.

The transparent layer can be produced by, e.g., MBE (Molecular BeamEpitaxy).

Alternatively, however, other deposition techniques, e.g., vacuumdeposition techniques (in particular chemical vapor depositiontechniques or physical vapor deposition techniques), can be used, suchas sputtering, atomic layer deposition, pulsed laser deposition.

On the other hand, MBE can make possible epitaxial, e.g., homoepitaxial,growth of the transparent layer. This can result in very lowresistitivies and interface resistances and good transparency.

The transparent layer can, accordingly, be “regrown” on the first layer.

Low resistances at interfaces between the first layer and thetransparent layer can be achieved when the transparent layer adopts thelattice structure and lattice constant from the first layer.

The transparent layer can have a high degree of crystallinity, which canresult in a low percentage of non-radiative recombination and thus inhigh conversion efficiency. E.g., a full-width at half maximum (FWHM) ofan Omega rocking curve in X-ray diffraction can be below 2° or ratherbelow 1°.

The low resistivities of the transparent layer mentioned above can beachieved by high point defect concentrations in the transparent layer.The point defects can be vacancies (i.e., missing atoms in the lattice)or can be impurities such as foreign atoms in the lattice, which can be,e.g., dopants. Point defect concentrations in the transparent layer canbe, e.g., at least 5·10¹⁹/cm³. They can be, e.g., up to 1·10²¹/cm³. Insome embodiments, the point defect concentration in the transparentlayer is between 5·10¹⁹/cm³ and 5·10²⁰/cm³.

The foreign atoms in the lattice of the transparent layer can be one ormore of Si, O, C, Mg, Be, Ge, Zn, Ti.

In some embodiments, the first layer is a p-doped layer, e.g., with Mgas dopant, and the transparent layer is an n-doped layer, e.g., dopedwith Si and/or with O. But also other foreign atoms may be present (cf.above).

In some other embodiments, the first layer and the transparent layer areboth p-doped layers, e.g., both with Mg as dopant.

For n-type transparent layers, it is, with technologies currently athand, easier to achieve high conductivities than for p-type layers, ashas been explained above.

The transparent layer can be used for efficient injection of chargecarriers from the transparent layer into the first layer or from thefirst layer into the transparent layer, in response to application of anelectric field across the transparent layer and the first layer. Theelectric field can extend across the semiconductor structure and thetransparent layer.

The transparent layer and the first layer are, in some embodiments, indirect contact with each other (and thus form a common interface) overthe full lateral extension of the transparent layer and/or of the firstlayer.

In order to better define the current flow in the semiconductorstructure, however, a current aperture stop can be provided. This canalso be beneficial if one or more electrical contacts are provided onone or more portions of the transparent layer, in particular in case aprecision with which the electrical contact(s) are laterally positionedis low, e.g., is lower than a precision with which the current aperturestop can be laterally positioned.

Thus, as an option, the device can comprise a current aperture stopbetween a portion of the semiconductor structure and a portion of thetransparent layer. In a lateral area defined by the correspondingaperture (i.e. by the opening defined by the current aperture stop), thefirst layer and the transparent layer establish their common interfacewhich has been mentioned above already. In this case, the commoninterface can be, e.g., extended also across a portion of thetransparent layer and, in instances, also across a portion of the firstlayer.

The current aperture stop can be provided for confining electricalcurrents flowing from the transparent layer through the first layer intothe active region or vice versa.

In some embodiments, the current aperture stop laterally encircles aportion of the first layer. In particular, it can do so in a commonvertical range in which both, the current aperture stop and the firstlayer are present.

For particularly low resistivity and high transparency of thetransparent layer, the transparent layer can be epitaxially (and evenhomoepitaxially) grown on the current aperture stop, as this can resultin particularly good crystallinity of the transparent layer.

The current aperture stop can be epitaxially or even homoepitaxiallywith the first layer. This can be achieved in various ways some of whichwill be explained.

In some embodiments, the current aperture stop is (e.g., epitaxially)grown on the first layer.

In some embodiments, the current aperture stop is created in the firstlayer. This can be accomplished, e.g., without having to grow anotherlayer establishing the current aperture stop. E.g., impurityconcentrations such as, e.g., co-dopant concentrations, can be locallychanged in order to produce the current aperture stop and/or a status(activated or passivated) of a dopant in the first layer can be locallychanged in order to produce the current aperture stop in the firstlayer.

It is also possible, e.g., to grow (e.g., regrow) on a layer of thesemiconductor structure, such as on the first layer, a resistivematerial (i.e., a material having a resistivity exceeding theresistivity of the first layer to establish the current aperture stop),possibly after etching of at least a part of the first layer in alateral region where the current aperture stop shall be established.

This will be described in more detail below.

The current aperture stop can be made of a (relatively) high resistivitymaterial. E.g., material comprised in the current aperture stop can havea resistivity of at least 10 times the resistivity of the first layer,or even of at least 100 times the resistivity of the first layer. E.g.,material comprised in the current aperture stop can have a resistivityof at least 5.10⁻² Ωcm or even of at least 1 Ωcm.

By means of the current aperture stop, a current effecting the lightemission in the semiconductor structure can be confined and/or the lightemission from the semiconductor structure can be confined.

The before-mentioned common interface of the transparent layer and thefirst layer can be surrounded, e.g., be fully encircled, by the currentaperture stop.

In some embodiments, the current aperture stop is present between thetransparent layer and the first layer (laterally outside an area takenby said common interface). This way, e.g., the first layer can have itsfull lateral extension where it is close to the active region of thesemiconductor structure. This can result in a high light output.

In some embodiments, the current aperture is present between an activeregion of the semiconductor structure and the transparent layer(laterally outside an area taken by said common interface). This way,e.g., active region (such as an i-layer of the semiconductor structureor the p-n-interface in case of a semicondcutor structure of p-n type)can have its full lateral extension. This can result in a high lightoutput.

For particularly low resistivity of the transparent layer in (lateral)regions where the transparent layer interfaces the current aperturestop, it can be beneficial to have the current aperture stop epitaxially(or even homoepitaxially) with the first layer and/or to have thetransparent layer epitaxially (or even homoepitaxially) with the currentaperture stop.

When the current aperture stop is epitaxial with the first layer, thebase on which the transparent layer is deposited can have the samecrystal properties where the first layer establishes said base (i.e. inthe lateral region of the current aperture) and where the currentaperture stop establishes said base. Accordingly, a low defect densitycan be achieved not only where the transparent layer interfaces thecurrent aperture stop and where it interfaces the first layer, but alsowhere the current aperture stop abuts the current aperture.

In some embodiments, the transparent layer laterally overlaps thecurrent aperture stop. One or more electrical contacts of the device(for applying currents for effecting the light emission) can furthermorebe present in a lateral region defined by this overlap, whereas the oneor more electrical contacts can furthermore be free from lateral overlapwith the current aperture. This way, light blocking by electricalcontacts can be avoided, in particular in case of top emission (throughthe current aperture, through the transparent layer).

Producing the current aperture can be accomplished, e.g., in one of thefollowing ways:

In a first embodiment, a dopant in the first layer is selectivelyactivated. This implies that before establishing the current aperturestop, the first layer contains a passivated dopant. This can be the casebecause of the way the first layer is produced. E.g., in case of aIII-nitride layer which is p-doped by Mg and produced in such a way thatthe Mg atoms are passivated by H (hydrogen), as it is the case, e.g.,when such a layer is conventionally produced by means of MOVPE. Or, apassivated dopant can be produced, e.g., by applying an additionalprocess step in which the dopant (which previously was not passivated)is passivated.

E.g., a diffusion-inhibiting mask can be selectively deposited on thefirst layer in the (lateral) region(s) where the current aperture stopshall be established, and then passivating atoms are removed from thenon-masked region(s) of the first layer such as by diffusion, e.g.,promoted by a heat treatment such as by heating to above 450° C. Thisway, a thickness of the current aperture stop identical with thethickness of the first layer can be achieved. Before applying thetransparent layer, the diffusion-inhibiting mask can be removed.

In the first embodiment, but also in second and third embodiments,device portions can be produced (namely the current aperture stop andthe first layer portion establishing the current aperture) which arelocated laterally next to each other (e.g., abutting each other) andlocated on the same vertical level, and which have differentresistivities. In case of the first embodiment, they can have stronglydifferent concentrations of passivated and activated dopant. E.g., whilein the current aperture, more than 40% or even more than 50% of thedoping atoms are activated, in the current aperture stop, less than 5%of the doping atoms are activated.

In a second embodiment, a portion of the first layer (with activateddopants) is converted into the current aperture stop by increasing aresistivity therein by locally introducing impurities (foreign atoms)into the first layer. The foreign atoms can be co-dopants, such asn-dopants, e.g., Si or O or Ge, in case the first layer is a p-layer,e.g., with Mg as dopant, or can be atoms which passivate the dopant(s)in the first layer, such as H atoms in III-nitrides which are p-doped byMg. Or they can be other types of atoms such as Ti, Ar, C, Cl whichincrease resistivity, e.g., of III-nitrides which are p-doped by Mg, bymechanisms which are today partially not yet completely understood.

In case the dopant(s) in the first layer is (are), after deposition,initially passivated, e.g., by H atoms passivating Mg atoms as dopant(cf. the example above with p-doped III-nitride), the dopant(s) can beactivated before the local introduction of impurities into the firstlayer. This can be accomplished, e.g., by the before-mentioned heattreatment.

In instances, a thickness of a current aperture stop produced in thesecond embodiment can be smaller than a thickness of the first layer;and accordingly in this case, the current aperture layer can be locatedbetween the first layer and the transparent layer.

In one variant of the second embodiment, a diffusion-promoting mask islocally applied to regions where the current aperture stop shall beestablished, wherein the mask contains said impurity atoms. By diffusionof the impurity atoms from the mask into the first layer, the currentaperture stop is created. In order to promote the diffusion of theimpurity atoms into the first layer, a heat treatment such as heating toabove 100° (and, optionally below 450° C.) can be applied. Examples fordiffusion-promoting masks are masks containing Ti (for Ti asimpurities), masks containing Ge (for Ge as impurities), maskscontaining SiN (for Si as impurities), masks containing SiO₂ (for Siand/or O as impurities). Before applying the transparent layer, thediffusion-promoting mask can be removed.

In another variant of the second embodiment, a diffusion-inhibiting maskcan be selectively deposited on the first layer in the (lateral) regionwhere the current aperture shall be established, and then, impuritiessuch as H, O, C, Si, Cl, Ar can be introduced into the first layer—inthe region(s) not protected by the diffusion-inhibiting mask and thus inthe region(s) where the current aperture stop shall be established.E.g., a plasma can be applied which contains one or more types of theimpurity atoms. The plasma can be applied, e.g., at between 0.001 mbarand 0.2 mbar (or between 1 mTorr and 100 mTorr) pressure and at plasmapower supply voltages between 50 V and 500 V. Before applying thetransparent layer, the diffusion-inhibiting mask can be removed.

In a third embodiment, material of the current aperture stop isdeposited on a portion of the semiconductor structure, e.g., on thefirst layer, so as to form the aperture stop, e.g. by means of MBE. Thematerial of the current aperture stop has a resistivity which isrelatively higher than the resistivity of the first layer. The materialcan be, e.g., intrinsic (non-intentionally doped) semiconductor material(e.g., a III-nitride) or co-doped material, e.g., containing a p-dopantsuch as Mg and one or more n-dopants such as Si, O. If in the depositionof the current aperture stop, material is also deposited in the(lateral) region of the current aperture such as on a removable maskpresent there, such material can be removed by removing the removablemask, e.g., in a lift-off technique.

The material of the current aperture stop in the third embodiment can beepitaxial and in particular even homoepitaxial with the material of thefirst layer.

Also for the third embodiment applies that in case the dopant(s) in thefirst layer is (are), after deposition, initially passivated, e.g., by Hatoms passivating Mg atoms as dopant (cf. the example above with p-dopedIII-nitride), the dopant(s) can be activated before the localintroduction of impurities into the first layer. This can beaccomplished, e.g., by the before-mentioned heat treatment.

In a first variant of the third embodiment, prior to depositing thematerial of the current aperture stop, a portion of the first layer isremoved in the (lateral) region(s) where the current aperture stop shallbe established, e.g., by etching such as ion etching, e.g., with Clions. It is even possible to completely remove the first layer. In orderto protect the (lateral) region(s) where the current aperture shall belocated, a protective mask such as an etch-protective mask (which can beidentical with or, alternatively, different from the removable maskmentioned above) can be applied to the first layer in said region(s).Then, material of the current aperture stop is deposited, so as to formthe aperture stop, e.g. by means of MBE. This way, it is possible toachieve that a surface of the current aperture stop on which thetransparent layer will be deposited lies in one and the same (lateral)plane as a surface of the first layer on which the transparent layerwill be deposited. This can contribute to minimizing dislocations in thetransparent layer.

In a second variant of the third embodiment, no removal of material fromthe first layer takes place prior to depositing the material of thecurrent aperture stop. This way, the surface on which the transparentlayer will be deposited can be a non-planar surface, but has a higherportion (on the current aperture stop) and a lower portion (on the firstlayer where it establishes the current aperture). Accordingly, thetransparent layer can have a curved cross-section. This can, ininstances lead to a somewhat increased density of dislocations of thetransparent layer.

In the context of the present patent application, it is assumed thatvariations in a ratio of activated and passivated dopants and variationsin point defect concentrations (such as concentrations of foreign atomsand/or concentrations of dopants and/or co-dopants) do not show aneffect on lattice parameters such as on the crystal structure and/or onthe lattice constant, at least for point defect densities up to2·10²¹/cm³.

In some embodiments, all layers of the semiconductor structure areIII-nitride layers.

In some embodiments, all layers of the current aperture stop areIII-nitride layers. The current aperture stop can be constituted by asingle structured layer.

In some embodiments, all layers of the transparent layer are III-nitridelayers. The transparent layer can be constituted by a single III-layer.

A composition of any of the III-nitride layers, in particular of thoseof the semiconductor structure, can vary along the vertical axis. E.g.,parameters x and y in Al_(x)Ga_(y)In_(1-x-y)N (with 0≦x≦1; 0≦y≦1) canvary along the growth axis in the first and/or in the second layer. Itis noted that III-nitride layers can be epitaxially grown on each othereven if they have different compositions in this sense.

In case the semiconductor structure includes an i-layer (notintentionally doped layer), the i-layer can comprise quantum wells.

In some embodiments, dopant concentrations in layers of the device, inparticular in the first and/or in the second layer, can vary along thevertical axis.

The transparent layer can be used as a vertical injector, i.e. as alayer via which current is vertically guided. Resistances to electricalcontacts (such as to metallic layers) can be minimized this way.

And/or the transparent layer can be used as a horizontal currentspreading layer, so as to ensure a good lateral distribution of currentsflowing through the semiconductor structure.

The transparent layer can be used as an efficient transparent currentinjector.

The transparent layer can be deposited after activation of a dopant inthe first layer grown by MOVPE such as a p-dopant, e.g., Mg. Thetransparent layer can contain high levels of impurity atoms (e.g.silicon, oxygen, carbon, magnesium, beryllium, germanium, zinc) or otherpoint defects (e.g., vacancies in the lattice structure of theIII-nitride, e.g., N-vacancies) and can be mono- or poly-crystallinedepending on technology and parameters used for growth/deposition.Epitaxial monocrystalline III-nitride (which, however, can contain somedegree of disclocations and of point defects) can make possible toachieve particularly low resistivities.

In some embodiments, the current aperture laterally overlaps with anactive region of the semiconductor layer.

In some embodiments, the device comprises a first electrical contact(which is electrically conductive, e.g., metallic) and a secondelectrical contact (which is electrically conductive, e.g., metallic):The first electrical contact can be present on (an be in direct contactwith) the transparent layer.

When an electric voltage is applied across the device, e.g., viaelectrical contacts of the device, charge carriers move in the followingsequence (or vice versa) through the following constituents of thedevice: transparent layer, first layer, (if present: i-layer betweenfirst and second layer), second layer.

We also describe a method for manufacturing a device, wherein the devicecomprises

-   -   a semiconductor structure operable to emit light from an active        region of the semiconductor structure and comprising a first        layer and a second layer; and in addition,    -   a transparent layer of a nitride of one or more group-III        elements, which is transparent to light emitted from the        semiconductor structure and which has an electrical conductivity        exceeding an electrical conductivity of the first layer;    -   and the method comprises producing, e.g., depositing, the        transparent layer on the first layer.

The device can be, e.g., any device herein described.

The first layer can be produced using metal organic vapor phase epitaxy(MOVPE). Also the second layer and, if present another layer between thefirst and the second layer such as a non-intentionally doped layer canbe produced by MOVPE.

The transparent layer, however, can be produced in a technique differentfrom MOVPE and in particular in a technique in which the first layer isnot exposed to hydrogen, such as using molecular beam epitaxy.

More details and embodiments relating to the method are described aboveand below.

Transparent layers can be produced which have the before-mentionedcommon interface with the first layer and which share an interface withthe current aperture stop. wherein the latter interface and said commoninterface are in one and the same plane. This way, the number ofdislocations in the transparent layer can be very small which can resultin low resistivities and/or high transparency.

Further embodiments and advantages emerge from the dependent claims andthe figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, the invention is described in more detail by means of examplesand the included drawings. The figures show schematically in across-sectional view:

FIG. 1 a device comprising a TCN layer and a current aperture stop;

FIG. 2A a prior art GaAs-based VCSEL;

FIG. 2B a prior art GaAs-based VCSEL with TCO current spreading layer;

FIG. 3 a device without current aperture;

FIG. 4 a flip chip LED device comprising a TCN layer;

FIG. 5A an LED device comprising a TCN layer and current aperture stop,with non-conductive substrate;

FIG. 5B an LED device comprising a TCN layer and current aperture stop,with conductive substrate;

FIG. 6 a ridge waveguide laser diode device;

FIG. 7 a stripe injection gain-guided laser device;

FIG. 8 an air-cladding laser device;

FIG. 9 a VCSEL device;

FIG. 10a device in which the current aperture stop extends from the TCNlayer to the active region of the semiconductor structure;

FIG. 11A a semiconductor structure on a substrate;

FIG. 11B the semiconductor structure of FIG. 11A with a current aperturein the first layer;

FIG. 11C a device obtained from the semiconductor structure of FIGS.11A, 11B by applying the TCN layer across the current aperture and aportion of the first layer.

DETAILED DESCRIPTION OF THE INVENTION

The described embodiments are meant as examples or for clarifying theinvention and shall not limit the invention.

We describe in several examples a transparent and conductive III-nitridelayer (herein also referred to as TCN layer or transparent layer) thatcan be present, e.g., on an uppermost Mg-doped layer of a III-nitridep-i-n junction grown by, e.g., conventional MOVPE.

FIG. 1 illustrates schematically such a device 10 in a cross-sectionalview.

Even though p-i-n type semiconductor structures are illustrated throughmost of the present description, corresponding embodiments with p-n typesemiconductor structures are also possible embodiments.

The TCN layer 5, grown as a monocrystalline layer (e.g. by MBE or MOVPE)or deposited as a polycrystalline layer (e.g., by sputtering,evaporation or atomic layer deposition), provides very low resistivity,in particular- to metallic contacts, and offers excellent transparencyto UV and/or visible light. The TCN layer 5 has a high concentration ofimpurities (like for example silicon, oxygen, carbon, magnesium,beryllium, germanium, zinc atoms) or other types of point defects, likefor example Nitrogen vacancies.

The semiconductor structure 4 is a p-i-n structure, e.g., grown byMOVPE. The p-doped layer 1 is also referred to as first layer 1, then-doped layer 2 is also referred to as second layer 2, and the layer 3in between can be an i-layer, i.e. a non-intentionally doped layer.

Semiconductor structure 4 is post-growth thermally annealed to activatethe p-dopant in the first layer 1, i.e. to remove hydrogen bonded to theMg dopant, and it can be also processed by conventional techniques(photolithography, etching, plasma passivation, ion implantation, etc.),e.g., to optionally create a current aperture 7 defined by a currentaperture stop 6 before the growth/deposition of the TCN layer 5.

III nitride based p-i-n structures 4 grown by MOVPE are currenttechnology required for producing highly efficient and long-lifetimelight-emitting diodes and laser diodes with emission in the UV orvisible part of the spectrum. Despite the strong degree of developmentthat such structures have experienced over the last 20 years, someaspects related to the quality of the contact layers, in particular tothe p-side of the junction, remain unsolved. The present descriptionrelates i.a. to the following aspects:

Low values for the specific resistance of metallic contacts provided forcurrent injection to the p-doped layers, even though highly desirable,are difficult to achieve. Most values reported in literature lie in thehigh 10⁻⁴ Ω·cm⁻² or even 10⁻³ Ω·cm⁻² range for metallic contactsdeposited over Mg-doped GaN in light-emitting diodes with visibleemission. UV-emitters with aluminium-containing uppermost layers sufferfrom even worse resistances. This resistance has a direct impact on thecurrent-voltage (I-V) characteristics of the final device, and alsoimpacts the electric-to-optic power conversion efficiency.

The known Mg-doped p-layers themselves have a considerable resistivity,which apparently renders impossible in known technology the realizationof device geometries with lateral injection (current flowing along alateral direction, i.e. in-plane), as it can be for example desirablefor intracavity Vertical Cavity Surface Emitting Laser (VCSEL) contactsin the GaAs material system. FIG. 2A illustrates a prior art GaAs-basedVCSEL. Metal contacts are referenced 8 and 9, reflectors are referenced11 and 12. The open arrow indicates the direction of light emission.This limitation has been overcome in prior art by the use of transparentconductive oxide layers (TCO) that can provide a decent specificresistance to the uppermost p-layer 1 in the epitaxial structure (e.g.,4 to 9 times 10⁻⁴ Ω·cm⁻²). And at the same time, it can offer a certaindegree of transparency. FIG. 2B illustrates such a device with a TCOlayer 15. Despite strong development efforts and use of suchsemi-transparent layers, it is a challenge in prior art to providecontact layers that are better matched to the uppermost p-layer 1 of theepitaxial structure and that at the same time offer a high degree oftransparency.

We herein describe the use of a III-nitride layer (in particular AlInGaNwith any possible composition of Al, In and Ga atoms) that is deposited,e.g., regrown or epitaxially grown on, e.g., a conventional p-i-nstructure produced by MOVPE.

The as grown MOVPE structure comes with p-layers that are not active dueto the magnesium being bond to hydrogen atoms that are widely present inthe epitaxial growth environment. In order to achieve current injectionand to properly operate the device, the p-dopant is activated bypost-growth thermal annealing and hydrogen segregation from thestructure resulting therefrom. The activation of the p dopant can beperformed, e.g., either in-situ inside the MOVPE-growth equipment orex-situ in a thermal annealing oven. After the activation of the Mgdopant, the semiconductor structure 4 can be processed to create one ormore current apertures and then produce the TCN layer 5 (cf. FIG. 1), orthe TCN layer 5 can be produced directly on first layer 1. The lattercase with no current aperture is illustrated in FIG. 3.

In FIG. 3 is also shown that the semiconductor structure 4 can be (inany embodiment) present on a substrate 18.

The TCN layer, e.g., a highly Si-doped GaN layer, can be grown by MBE,e.g., over a conventional p-i-n structure 4. Nevertheless, other growthor deposition techniques such as MOVPE, sputtering, evaporation can, ininstances be used, too. In contrast to MOVPE as usually applied, MBE canbe carried out in a hydrogen-free environment. Therefore, using MBE orother hydrogen-free techniques for depositing the TCN layer 5 canprevent that Mg which is already present (in the first layer 1) isdeactivated (by hydrogen present during deposition of the TCN layer 5).Moreover, the TCN layer 5 can be epitaxially grown over the existingstructure (which includes the first layer 1 and, if present, also thecurrent aperture stop 6) and thus offers an excellent crystalline purityand can therefore be designed for very high transparency to the lightemitted from the active region of the underlying semiconductor structure4, i.e. from the i-layer in case of a p-i-n structure 4.

Light emitting diodes (LEDs) and laser diodes (LDs) can be designed withthe TCN layer 5, e.g., regrown by means of MBE. On such devices, theinventors already have achieved contact resistances as low as 1·10⁻⁶Ω·cm⁻² (which is two orders of magnitude lower than in conventionaldevices without TCN layer 5), while no adverse effect on the opticalperformance or quality of the devices has been found.

On the TCN layer 5 of the device 10, one or more electrical contacts(such as prior art contact 8, cf. FIGS. 2A, 2B) can be applied.

Examples of Possible Device Designs Including a TCN Layer 5

The device 10 can be, e.g., one of the devices sketched in thefollowing. The meaning of the reference symbols can be inferred from theprevious Figures, where not explicitly explained. The open arrowsindicate directions of light emission. Electrical contacts 8, 9 can bemetallic contacts.

Light Emitting Diode

-   -   a. FIG. 4 illustrates flip chip device 10: Herein, the TCN layer        5 can contribute to minimizing the resistance to the electrical        contact 8 which is used as reflector 13.    -   b. FIGS. 5A and 5B illustrate examples with current apertures 7,        in which a transparent contact geometry is realized such that        light is extracted through the TCN layer 5. This can contribute        to achieving very high transparency and homogeneous current        spreading over the p-layer (first layer 1) grown by MOVPE. FIG.        5A illustrates a case with a non-conductive substrate 18 a, FIG.        5B with a conductive substrate 18 b. Current aperture stop 6 can        be, but need not be present.

Edge-Emitting Laser Diode or Optical Amplifier or Superluminescent Diode

-   -   a. FIG. 6 illustrates a ridge waveguide laser diode device 10.        The TCN layer 5 can contribute to minimizing the resistance of        the p-side metallic contact 8 used to inject current into the        active region 3. A dielectric 17 is provided. The direction of        light emission is perpendicular to the drawing plane.    -   b. FIG. 7 illustrates a stripe injection gain-guided laser        device 10. The TCN layer 5 can contribute to minimizing the        resistance to the metallic contact 8. And it can contribute to        making the current injection uniform over a stripe-shaped        region. The stripe region can be defined by standard fabrication        techniques in the p-i-n structure grown by MOVPE before the        deposition of the TCN layer 5.    -   c. FIG. 8 illustrates an air-cladding laser device 10. The TCN        layer 5 is used in this case as a current spreading layer in a        p-i-n laser structure 4 designed for a large penetration of the        optical mode towards the p-side. The inset illustrates the modal        intensity I as a function of the coordinate −z. The interface        between air and the TCN layer 5 squeezes the laser mode back        into the active region 3 and can contribute to maximizing the        optical confinement. A stripe region can be defined by standard        fabrication techniques in the p-i-n structure grown by MOVPE        before the deposition of the TCN layer 5.

Vertical Cavity Surface Emitting Laser

-   -   a. FIG. 9 illustrates a VCSEL device 10 in which the TCN layer 5        is used as a current spreading layer to achieve efficient        intracavity injection of the VCSEL active region 3. It can be of        advantage for the operation of the VCSEL when the TCN layer has        a high transparency (such as more than 99%). A current aperture        (not illustrated in FIG. 9) can be defined by standard        fabrication techniques in the p-i-n structure 4 grown by MOVPE        before the deposition of the TCN layer 5. Reflectors 11, 12 are        provided.

Manufacturing Details and Further Embodiments

In several examples above with a current aperture stop 6, the firstlayer 1 extends below the current aperture stop 6. However, this needsnot be the case, neither in the embodiments above, nor in those below.In FIG. 10, an embodiment is illustrated in which a thickness of thecurrent aperture stop 6 is identical to a thickness of the first layer1. While in illustrated embodiments above, only a portion of the firstlayer 1 is present in the lateral area defined by the current aperture7, in FIG. 10, the whole first layer is present in the lateral areadefined by the current aperture 7.

FIGS. 11A, 11B, 11C illustrate method steps for manufacturing a device10.

Initially provided/manufactured is the semiconductor structure 4 (whichalso can be considered the body of the light emitting device 10). Anexample is illustrated in FIG. 11A. It includes one or more lightemitting layers that are sandwiched between doped layers 1, 2 ofdifferent type. P-type layers can be above, towards the surface of thedevice. N-type layers can be below, in between the light emitting layersand an optional substrate 18. Substrate 18 can be, e.g., free-standingIII-nitride (e.g., GaN), sapphire, Silicon, SiC. It may includereflectors, in particular optical reflectors having a high reflectivitysuch as Bragg reflectors In FIG. 11A, i-layer 3 represents the activeregion of the semiconductor structure 4.

In a next step, one or more current aperture stops 6 can be produced(cf. FIG. 11B), in particular in the semiconductor structure 4 and evenmore specifically in the first layer 1. As illustrated in FIG. 11B, acurrent aperture 7 can be located within the top p-layer 1. Currentaperture 7 can have lower resistivity than the surrounding areas, i.e.than the current aperture stop 6. In the current aperture stop,conductivity due to the p-type dopant can be, e.g., compensated by thepresence of other impurities of different type, i.e. of n-type, like forexample Si, Oxygen, Titanium, Carbon. Or the p-type dopant (Mg) can bepassivated by Hydrogen, in order to form the current aperture stop inthe first layer 1.

A current aperture stop 6 can also be considered a current confiningarea or current confining structure or current blocking structure.

In a next step (cf. FIG. 11C), the TCN layer 5 is applied on top. Thep-i-n structure 4 (more specifically a portion of the first layer 1) andthe current aperture stop 6 are overgrown by the TCN layer, and thus bya transparent highly n-type or p-type doped III-layer (the group-IIIelements being one or more of Al, In, Ga). The TCN layer 5 minimizes theresistance to metallic layers that can be deposited thereabove. And itcan also provide a tunnel junction at the interface with the uppermostlayer (first layer 1) for efficient current injection through thenon-resistive current apertures, in case the TCN layer 5 is n-type dopedand the first layer is p-type doped or vice versa.

The body of the light emitting device (semiconductor structure 4, cf.FIG. 11A) can be made by, e.g., MOCVD or MBE.

The current aperture 7 can be formed, e.g., from a first layer 1 whichis a resistive p-layer, by decreasing the resistivity in local areas(where the current aperture shall be located), e.g., by removal of atype of impurities (like for example Hydrogen or Oxygen impurities thatare present in the layer together with the Mg).

Or, the current aperture can be formed, e.g., from a low-resistivityfirst layer which is a p-layer, by increasing the resistivity in localareas (where the current aperture stop shall be located), by theintroduction of doping species of different type (i.e. n-type, like Si,O₂, Ti, C), or by locally passivating the p-type Mg with Hydrogen.

The highly doped TCN layer 5 (e.g., n-type doped; or p-type doped) canbe epitaxially deposited by MBE. In case of an n-doped TCN-layer 5, thiscan result in an efficient tunnel junction at the interface with theuppermost p-layer.

Some details of exemplary current aperture stops:

The current aperture seen from the top of the wafer (i.e. in verticaldirection) can have, e.g., a circular or rectangular shape.

-   -   a. Circular: diameter can be, e.g., between 2 μm and 100 μm;    -   b. Rectangular: Dimension in x: can be, e.g., between 2 μm and        100 μm; and dimension in y: can be, e.g., between 2 μm and 2000        μm

A device can include several current apertures. E.g., a current aperturecan be repeated several times within a single device, e.g., to producelinear or 2D current aperture arrays.

The thickness t (along the vertical axis) of the current aperture stop 6(resistive region) can be, e.g., between 10 nm and 1 μm or between 10 nmand the full thickness of the first layer. Its thickness can bedependent on the method used for creating the current aperture stop:

-   -   a. Selective Mg activation: Thickness t can be identical with        the full thickness of the first layer 1    -   b. Local passivation by mask and annealing: e.g., 50 nm<t<500 nm    -   c. Local passivation by plasma treatment: e.g., 5 nm<t<100 nm    -   d. Resistive region created by deposition (e.g., regrowth), such        as after etching: e.g., 10 nm<t<full thickness of the first        layer 1.

In the following, some ways for producing a current aperture stop willbe briefly described (first layer p-doped with Mg assumed).

-   -   1. Selective activation by mask deposition and annealing:        -   Mask selectively deposited only over regions outside current            aperture 7. Mask can be made of, e.g., SiN, or SiO₂.        -   Mg activation by thermal annealing: T>450° C.        -   Removal of mask    -   2. Selective passivation by mask deposition and annealing:        -   Mg activation under thermal annealing, e.g., at above >450°            C.        -   Mask selectively deposited only over regions where            resistivity will be increased, i.e., where current aperture            stop 6 shall be created. Mask can be made of, e.g., SiN, or            SiO₂ or Ti.        -   Annealing, e.g., at temperature −100° C.<T<450° C.        -   Removal of mask    -   3. Local passivation by plasma treatment:        -   Mg activation under thermal annealing, e.g., at above >450°            C.        -   Mask selectively deposited (e.g., a photoresist material) to            protect current aperture region.        -   Plasma treatment: plasma power supply typically in the 50 V            to 500 V range. Pressure: 1<p<100 mTorr. Possible atomic            species in the plasma: one of more of H, O, C, Si, Cl, Ar        -   Removal of mask    -   4. Resistive region created by etching and regrowth:        -   Mg activation under thermal annealing, e.g., at above >450°            C.        -   Mask selectively deposited to protect current aperture            region.        -   Etching down exposed surface to a desired thickness, e.g.,            by ion etching techniques including Cl atoms        -   Regrowth of, e.g., codoped (Mg and Si dopants), resistive            region(s), e.g., by MBE        -   Removal of mask

In all above cases, deposition of the TCN layer can take place afterremoval of the mask.

Some further exemplary details of devices 10 (cf., e.g., FIG. 3; but thedetails apply also if a current aperture stop 6 is present):

-   -   The substrate 18 is suited for epitaxial growth of an AlInGa—N        compound, and is, e.g., bulk GaN (thickness between 0.2 mm and        1 mm) or sapphire (thickness between 0.2 mm and 1 mm). In the        case of bulk GaN, the substrate is, e.g., n-doped with Si or O        impurities as dopant.    -   The second layer 2 can be made of a III-nitride crystal and        e.g., can contain any possible combination of the group-III        elements Al, Ga, In. The composition can change along the        vertical axis to create desired effects, e.g., for improving        light confinement and/or to optimize current injection towards        the active region 3 and/or for improved structural strain        management.    -   The second layer 2 can be, e.g., n-doped with Si. The doping        level can vary over the vertical axis, and is typically lower        closer to the active region 3 (e.g., between 1·10¹⁷/cm³ and        1·10¹⁸/cm³) and higher towards the substrate (e.g., more than        1·10¹⁸/cm³). The thickness of the second layer 2 can depend on        the device geometry. It can be, e.g., between 0.2 μm and 5 μm.    -   The active region 3 is typically non intentionally doped        (n.i.d.) and can be made of or at least comprise quantum wells,        such as lower band-gap III-nitride crystal material (such as one        or multiple nitride layers containing In and Ga and Al) embedded        in a larger bandgap group-III-nitride crystal material acting as        a barrier (such as one or multiple III-nitride layers containing        In and Ga and Al). The thickness of the active region 3 can        depend on the device geometry. It can be, e.g., between 2 nm and        500 nm.    -   The first layer 1 can be made of a III-nitride crystal and e.g.,        can contain any possible combination of the group-III elements        Al, Ga, In. The composition can change along the vertical axis        to create desired effects, e.g., for improving light confinement        and/or to optimize current injection towards the active region 3        and/or for improved structural strain management.    -   The first layer 1 can be, e.g., p-doped with Mg. The doping        level can vary over the vertical axis, and can be lower closer        to the active region 3 (e.g., between 1·10¹⁸/cm³ and 5·10¹⁸/cm³)        and higher towards the substrate (e.g., more than 5·10¹⁸/cm³ and        can be lower than 2·10²⁰/cm³). The thickness of the first layer        1 can depend on device geometry. It can be, e.g., between 0.05        μm and 1 μm.    -   The semiconductor structure 4 (body of the light emitting        device), being the total of first layer 1 plus active region 3        plus second layer 2 can be epitaxially grown by Metal Organic        Vapour Phase Epitaxy (MOVPE) or MOCVD (Metal Organic Chemical        Vapor Deposition).    -   The transparent layer 5 (Transparent Conductive Nitride layer,        TCN layer) is in contact with the first layer 1 and can be grown        by MBE. It can be highly n-doped, e.g., with Si. Typical        n-doping levels can be between 1·10¹⁸/cm³ and 5·10²⁰/cm³. The        dopant concentration can vary along the vertial axis. Typical        thickness t of the TCN layer 5 can depend on the device geometry        and amount to, e.g., between 30 nm and 300 nm.

Summarizing some points of the described devices and methods, we notethat MOVPE can make possible to produce III-nitride based semiconductorstructures of excellent crystal quality capable of high light emissionefficiency, but Mg atoms (as p-dopant) are passivated by hydrogen suchthat as-grown structures have a high resistivity (in the p-doped layer).The hydrogen can be removed by annealing above about 400° C., resultingin a good conductivity. But due to relatively low hole mobility in an(activated) Mg-doped first layer, lateral current injection isproblematic (because of still relatively high resistivity), andproducing ohmic contacts of low resistance on the first layer isproblematic, too. The deposition of the transparent layer 5 (TCN layer)can contribute to achieving a good lateral current spreading andlow-resistance ohmic contacts.

Provision of current aperture stops, e.g., as described, can provide theeffects as described. It can be produced in a way that it is epitaxialwith the first layer, cf. above.

With or without current aperture stop 6, the transparent (TCN) layer canbe grown without exposing the first layer 1 to hydrogen, e.g. when usingMBE. Epitaxial deposition of the transparent layer 5 (on the first layer1) can result in low resistances at the interface to the first layer andto low resistivity in the transparent layer 5. And in case a currentaperture stop 6 is present, epitaxial deposition of the transparentlayer (on the current aperture stop) can result in low resistances atthe interface to the current aperture stop 6.

And the interfaces of the transparent layer to the first layer and tothe current aperture stop can both be planar and lie both in one and thesame (lateral) plane.

High doping levels of the transparent layer, e.g., with Si as dopant,can result in excellent electrical properties.

The foregoing description and accompanying drawings illustrate theprinciples, preferred embodiments and modes of operation of theinvention. However, the invention should not be construed as beinglimited to the particular embodiments discussed above. Additionalvariations of the embodiments discussed above will be appreciated bythose skilled in the art.

Therefore, the above-described embodiments should be regarded asillustrative rather than restrictive. Accordingly, it should beappreciated that variations to those embodiments can be made by thoseskilled in the art without departing from the scope of the invention asdefined by the following claims.

What is claimed is:
 1. A device comprising: a light-emittingsemiconductor structure operable to emit light and comprising a firstlayer, which is p-type and composed of a nitride of at least onegroup-III element, and a second layer, which is n-type and composed of anitride of at least one group-III element; and a transparent, currentspreading layer of a nitride composed of at least one group-III element,which is transparent to light emitted from the light-emittingsemiconductor structure and of sufficiently high electrical conductivityto provide lateral spreading of injection current for the light-emittingsemiconductor structure within the transparent, current spreading layer.2. The device according to claim 1, wherein the electrical conductivityof the transparent, current spreading layer exceeds the electricalconductivity of the first layer by at least a factor of
 10. 3. Thedevice according to claim 1, wherein the transparent, current spreadinglayer is n-type.
 4. The device according to claim 1, further comprisinga current aperture stop defining a current aperture between a portion ofthe first layer and a portion of the transparent, current spreadinglayer.
 5. The device according to claim 4, wherein the current aperturestop has a resistivity of at least 10 times a resistivity of the firstlayer.
 6. The device according to claim 4, wherein the current aperturestop is composed of a nitride of at least one group-III element.
 7. Thedevice according to claim 1, wherein the first layer is doped with Mg asa p-type dopant.
 8. The device according to claim 1, wherein a pointdefect density in the transparent, current spreading layer is at leastone of: above 5·10¹⁹/cm³; between 5-10¹⁹/cm³ and 1·10²¹/cm³³; andbetween 5·10¹⁹/cm³ and 5·10²⁰/cm³.
 9. A method for manufacturing adevice, the method comprising: producing a light-emitting semiconductorstructure operable to emit light by depositing a first layer, which isp-type and composed of a nitride of at least one group-III element, anddepositing a second layer, which is n-type and composed of a nitride ofat least one group-III element; and depositing a transparent, currentspreading layer, which is n-type and composed of a nitride of at leastone group-Ill element, the transparent, current spreading layer beingconfigured to be transparent to light emitted from the light-emittingsemiconductor structure and of sufficiently high electrical conductivityto provide lateral spreading of injection current for the light-emittingsemiconductor structure within the transparent, current spreading layer.10. The method according to claim 9, wherein at least the first layer ofthe light-emitting semiconductor structure is deposited using metalorganic vapor phase epitaxy, and wherein the transparent, currentspreading layer is deposited using molecular beam epitaxy.
 11. Themethod according to claim 9, wherein as deposited the first layer haspassivated dopants, the method further comprising activating saidpassivated dopants in at least a portion of the first layer.
 12. Themethod according to claim 11, wherein the passivated dopants areactivated in said portion of the first layer by applying a local heattreatment.
 13. The method according to claim 9, further comprisingproducing a current aperture stop between the transparent, currentspreading layer and die first layer so as to define a current aperturetherebetween.
 14. The method according to claim 13, wherein the currentaperture stop has a resistivity amounting to at least 10 times aresistivity of the first layer.
 15. The method according to claim 13,wherein the current aperture stop is produced from a part of the firstlayer by locally increasing the resistivity in said part of the firstlayer, wherein locally increasing the resistivity in said part of thefirst layer comprises effecting a diffusion of foreign atoms locallyinto the first layer.
 16. The method according to claim 15, whereineffecting the diffusion of the foreign atoms locally into the firstlayer comprises: depositing on the first layer, in a lateral area wherethe current aperture stop is to be produced, a diffusion-promoting maskcontaining the foreign atoms; and effecting the diffusion of the foreignatoms into the first layer by applying a heat treatment.
 17. The methodaccording to claim 15, wherein effecting the diffusion of the foreignatoms locally into the first layer comprises: depositing on the firstlayer, in a lateral area where the current aperture is to be produced, adiffusion-inhibiting mask; and applying a plasma containing the foreignatoms to diffuse into the first layer.
 18. The method according to claim9, wherein the first layer is doped with Mg as a p-type dopant.
 19. Themethod according to claim 9, wherein the electrical conductivity of thetransparent, current spreading layer exceeds the electrical conductivityof the first layer by at least a factor of
 10. 20. The method accordingto claim 9, wherein a point defect density in the transparent, currentspreading layer is at least one of: above 5·10¹⁹/cm³: between 5·10¹⁹/cm³and 1·10²¹/cm³³; and between 5·10¹⁹/cm³ and 5·10²⁰/cm³.